The membrane changes involved in the developmentally programmed decrease in cell adhesiveness that accompanies sea urchin embryo primary mesenchyme formation are being subjected to ultrastructural and biochemical analysis. Specific projects in progress include: 1. Micromere isolation differentiation of primary mesenchyme in vitro. 2. Detailed electron microscopic study of dissociation of embryos and the reaggregation of the resulting single cells. 3. Lectin agglutinability of developing primary mesenchyme cells. 4. Identification of lectin receptors on developing primary mesenchyme cells. 5. Galactose oxidase labeling of intact developing primary mesenchyme cells.